1. Field of the Invention
The present invention relates generally to a sequence control system and its method of controlling sequential operation of machine tools, for instance and more specifically to a sequence control system and its method which can facilitate operation modification and trouble repair, in particular.
2. Description of the Prior Art
Recently, programmable sequence control systems are widely adopted in order to control various industrial machines in various industrial fields. The reason why these sequence control systems have come into wide use is that any required system can be configured by forming user programs; machine operation can be simply modified to required motions by changing only programs; and further the system cost is relatively low.
In the sequence control system as described above, however, since the processing method is a relay ladder method, when adopted to a very large-scale industrial machine, there exist problems in that the programs for the sequence control system for controlling the industrial machine is huge and complicated and further it takes a lot of time whenever the programs are modified. For example, when some member becomes unmovable, conditions under which the member is moved is checked in sequence on the basis of ladder diagrams. However, if the conditions are huge, it takes much time to check and find out the causes of the trouble. This is because it is impossible to understand the operation only by looking at the ladder diagrams.
To overcome the above-mentioned drawbacks involved in relay ladder method, recently sequence control systems of a graph set processing method have been adopted. In the sequence control systems the graph set method, since processing steps can be entered in correspondence to the machine operation, when the machines are modified or repaired, the processing steps stored in the sequence control system are outputted to understand machine operation with reference to the outputted processing steps. Therefore, it is possible to repair or modify the machines more easily as compared with those of relay ladder method.
For instance, when a machine which functions as shown in FIGS. 1 and 1(A) is controlled by a sequence control system of graph set method, first, a timing chart as shown in FIG. 1(B) is formed, and processing steps as shown in FIG. 1(C) are inputted in correspondence to the timing chart.
That is, first Y1 is turned on to raise a lifter (in step 1). If the lifter is raised to an upper limit and therefore a transition condition X1 is on, Y1 is turned off and Y3 is turned on and simultaneously Y5 is turned on to move clamps 1 and 2 (in steps 2 and 3). Succeedingly, transition conditions X3 and X5 are turned on, and therefore transition conditions of workpiece A is turned on; that is, when the clamps 1 and 2 move to the forward end and also the workpiece is A, Y3 and Y5 are turned off simultaneously and Y7 is turned on (in step 4). When the workpiece A is processed in a predetermined process and therefore transition conditions X7 is turned on, Y7 is turned off and Y8 is turned on (in step 5). On the other hand, when conditions X3 and X5 are turned on and further transition condition of workpiece B is turned on; that is, when the clamps 1 and 2 move to a forward end and the workpiece is B, Y3 and Y5 are turned off simultaneously and Y9 is turned on (in step 6). When the workpiece B is processed and therefore the transition condition X9 is turned on, Y9 is turned off and Y10 is turned on (in step 7). Thereafter, when the workpiece A or B has been processed and therefore the transition conditions X8 and X10 are turned on, Y8 and Y10 are turned off and Y4 is turned on and simultaneously Y6 is turned on to return the clamps 1 and 2 (in steps 8 and 9). Further, when the clamps 1 and 2 are returned and therefore transition conditions X4 and X6 are turned on, Y4 and Y6 are turned off and Y2 is turned on to lower the lifter (in step 10). When the lifter is lowered to a lower limit and therefore transition condition X2 is turned on, Y2 is turned off (in step 11).
As described above, in the sequence control system of graph set method, since it is possible to directly input processing steps on the basis of the timing chart as shown in FIG. 1(B), the system can be modified or repaired relatively simply.
In the sequence control system of graph set method as described above, although there are many advantages as long as the machine is controlled automatically, in case trouble occurs with the machine and therefore some members are required to be moved manually, various problems develop.
For instance, in the flowchart shown in FIG. 1(C), in case the lifter will not rise to an upper limit due to some trouble, the step 1 is kept as it is indefinitely. Since it is impossible to repair the machine under these conditions, a step for forcedly stopping this processing is additionally required. In other words, it is necessary to switch the automatic operation to the manual operation to move the lifter manually. However, if the flowchart is previously formed together with the manual operation under consideration of trouble, since the manual operation flowchart must be added for each process, the flowcharts for covering the entire operation are very complicated, and therefore there exists a problem in that the advantages of graph set method can not be utilized in practice.
Further, even if elements of the machine can be moved by the above-mentioned manual operation, it is impossible to restart the machine by simply switching the manual operation to the automatic operation after repair or adjustment. This is because machine elements are kept stopped at positions determined when the automatic operation has been switched to the manual operation.